25.8
电容器的介电极化
电容器中电介质的存在不仅会改变电压和电容，还会影响电场。 一般来说，电介质可以有两种类型：极性和非极性。 在极性电介质中，分子中的正电荷和负电荷分开一定距离，因此具有永久偶极矩。 相反，非极性电介质中不存在这种电荷分离，但是非极性分子在外部电场存在的情况下会被极化。 在这两种情况下，偶极子都是随机定…
电容器中的电介质层可由极性或非极性分子构成。
极性电介质中的正负电荷具有净分离，因此具有永久极化。非极性电介质没有电荷分离；然而，在外加电场存在时，这些电介质会被极化。
在完全充电的电容器中，若插入极性电介质，随机取向的偶极子会与电场对齐，从而在表面感应出等量且相反的电荷。
对于非极性电介质，电场会使正负电荷分离，从而产生极化，并进一步使它们沿电场方向排列。
外加电场与感应表面电荷产生的电场的矢量和即为净电场。该净电场与外加电场之间的关系由介电常数决定。通过重新整理各项，可得到感应电场的表达式。
当电场强度超过一定限度时，会使电介质中的分子发生电离。此时电介质表现出类似导体的性质，允许电流在极板之间通过，从而导致电介质击穿。
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Q1: What is the difference between polar and nonpolar dielectrics?
Polar dielectrics have permanent charge separation in their molecules, creating a permanent dipole moment. Nonpolar dielectrics lack this charge separation but become polarized when exposed to an external electric field. In both cases, dipoles are randomly oriented without an applied field, but polar molecules align more readily with the field.
Q2: How does an electric field cause polarization in nonpolar molecules?
When an external electric field is applied to a nonpolar dielectric, it separates the positive and negative charges within the molecules, inducing polarization. This induced polarization aligns the charges along the field direction. The nonpolar molecules effectively become polar in the presence of the external field.
Q3: What happens to the electric field when a dielectric is inserted into a capacitor?
The induced surface charges in the dielectric create an induced electric field that opposes the external field. The net electric field is the vector sum of the external and induced fields, which is weaker than the original external field. This relationship is quantified using the dielectric constant.
Q4: What are the effects of induced surface charges on capacitor performance?
Induced surface charges on the dielectric faces reduce the voltage across the capacitor plates and increase the overall capacitance. These charges create an induced electric field that opposes the external field, effectively weakening the net field between the plates and enhancing the capacitor's ability to store charge.
Q5: What is dielectric breakdown and when does it occur?
Dielectric breakdown occurs when the external electric field magnitude exceeds a critical limit, ionizing molecules in the dielectric material. This ionization produces free electrons, allowing current to flow between the capacitor plates through the dielectric. The material then behaves like a conductor, compromising the capacitor's function.
Q6: How do polar dielectrics respond when placed in a uniform electric field?
Polar dielectrics have randomly oriented dipoles that align with the applied uniform electric field. This alignment induces equal and opposite surface charges on the dielectric faces. The organized dipole orientation reduces the net electric field between the capacitor plates and increases capacitance.
Q7: How is the net electric field related to the external field in a dielectric?
The net electric field in a dielectric is the vector sum of the external field and the induced field from surface charges. This relationship is expressed mathematically using susceptibility permittivity and dielectric constant, which quantifies how much the material reduces the electric field compared to vacuum.